A polarizing plate has been generally used as a structure where a protection film is laminated on one surface or both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichromatic dye or iodine by using an adhesive. In the related art, a triacetyl cellulose (TAC)-based film has been mainly used as the protection film for the polarizing plate, but there is a problem in that the TAC film is easily deformed under high temperature and high humidity environment. Accordingly, recently, protection films of various materials which can replace the TAC film have been developed, and for example, a method where polyethylene terephthalate (PET), cycloolefin polymer (COP), and acryl-based films, and the like are used alone or are used while being mixed is proposed.
Meanwhile, the protection film for the polarizer may include various functional coating layers such as an antireflection layer and a hard coating layer on an opposite surface of a surface to which the polarizer is attached for the purpose of reflection prevention, durability improvement, scratch prevention, visibility improvement, and the like. The functional coating layers are generally formed by a method of applying a coating composition including a base resin, a solvent, an additive, and the like on the protection film, and then performing curing. However, there is a case where adhesion force with the functional coating layer is not sufficient according to a kind of protection film, and in this case, there is a problem in that the functional coating layer is stripped from the protection film or damaged to reduce performance of the polarizing plate.
In order to solve the aforementioned problem, a method of performing surface treatment such as plasma treatment and corona treatment on a surface of the protection film or forming a primer layer, and the like have been proposed, but by the method proposed until now, adhesion force between the protection film, particularly an acryl-based protection film, and the functional coating layer cannot be sufficiently ensured. For example, in the case of a urethane-based resin proposed as a primer layer for a protection film in the related art, there are problems in that since water resistance and solvent resistance are low, adhesion force is reduced due to permeation of moisture during storage over a long period of time under high humidity, or in the case where the coating composition including the organic solvent is applied on the primer layer, the primer layer is dissolved due to the organic solvent included in the coating composition to be detached from the film.
Moreover, in the case of the functional coating layer for reflection prevention and scratch prevention, diffused reflection occurs by using silicas having different sizes to reduce reflectance and thus provide an antireflection function, or reflectance is reduced to 1% s by using a layer having a low refractive index to provide the antireflection function. Likewise, there is a problem in that in the case where silicas having different sizes are used or the layer having the low refractive index is used, the coating layer is transparent, and thus in the case where the refractive index difference between the primer layer and the acryl film is large, a rainbow phenomenon due to interference of light occurs.
Accordingly, in order to solve the aforementioned problems, an optical film having excellent adhesion force and optical property through a primer layer having excellent water resistance and solvent resistance, and a polarizing plate need to be developed.